H. Arenhövel

Electrodisintegration of deuterium including nucleon detection in coincidence

Abstract A consistent nonrelativistic treatment of deuteron electrodisintegration is presented including effects from meson exchange currents and isobar configurations. The 2H(e, e′) cross section for an oriented and an unoriented deuteron target is given in terms of form factors while the 2H(e, e′p) and 2H(e, e′ p ) coincidence cross sections are described by proper structure functions. The T-matrix is calculated by a multipole expansion up to L = 6 for inclusion of the final state interaction while for all higher multipoles the Born approximation for the final state has been used and very good convergence is achieved. We find good agreement between theory and experiment for 0 ≦ q2 ≦ 20 fm−2 and excitation energies up to π-threshold. Coincidence experiments at low q but higher excitation energies turn out to be particularly sensitive to interaction effects from virtual isobar excitation.

Double-pion photoproduction on nucleon and deuteron

Abstract.Photoproduction of two pions on nucleon and deuteron is studied for photon energies from threshold up to Eγ = 1.5 GeV. For the elementary operator an effective Lagrangian approach is used with resonance and Born contributions. The model parameters are fixed by resonance decay widths and multipole analyses of single-pion photoproduction. A satisfactory description of total cross sections of two-pion production on the proton for various charge channels is achieved, except for π0π0 production for which a significant underestimation is found near threshold. The operator then serves for the evaluation of this reaction on the deuteron in the impulse approximation. In addition, NN rescattering in the final state is taken into account, but πN and ππ rescatterings are neglected. Results are presented for total cross sections and target asymmetries.

A GAUGE INVARIANT EXCHANGE CURRENT FOR THE PARIS POTENTIAL

Abstract A gauge-invariant exchange-current model is constructed for the parametrized form of the Paris potential. The influence of this current is studied for photo- and electro-disintegration of the deuteron. A comparison with a regularized π-exchange current shows significant differences for the γ-asymmetry in photodisintegration, for the transverse interference structure function in electrodisintegration and also for d(e, e′)np near threshold at higher momentum transfer. But further inclusion of a regularized ρ-exchange current with proper parametrization almost identical to the ρ-parameters of the Bryan-Gersten potential leads to results very similar to the present calculation.

Measurement of the electric form factor of the neutron at Q 2 = 0.3-0.8 (GeV/ c) 2 ⋆

Abstract.The electric form factor of the neutron, GE,n, has been measured at the Mainz Microtron by recoil polarimetry in the quasielastic D(¯e, e’¯n)p reaction. Three data points have been extracted at squared four-momentum transfers Q2 = 0.3, 0.6 and 0.8 (GeV/c)2. Corrections for nuclear binding effects have been applied.

Influence of final-state interaction on incoherent pion photoproduction on the deuteron in the region of the

Abstract:The influence of final-state NN and πN rescattering in incoherent pion photoproduction on the deuteron has been investigated. For the elementary photoproduction operator an effective Lagrangian model is used which describes well the elementary reaction. The interactions in the final two-body subsystems are taken in separable form. While NN rescattering shows quite a significant effect, particularly strong for neutral pion production, πN rescattering is almost negligible. Inclusion of such effects leads to an improved and quite satisfactory agreement with experiment.

\mth{\Delta}

The size of exchange corrections to electric transitions is investigated for deuteron photodisintegration. Large contributions are found in El transitions, which become dominant above 40 MeV. Most of them are already included by using Siegerts theorem together with classical one-body operators. Higher multipoles are only little affected except for rather high energies. The Born approximation for the final states gives in general a poor description.

-resonance

Abstract The (NΔ) system has been investigated to see whether a quasi-bound state exists for which the lifetime of the Δ is considerably increased. The (NΔ) interaction using π- and ρ-meson exchange has been studied for various approximations. In particular, the Δ-nucleon mass difference and the Δ-width have been included leading to a complex (NΔ) potential. One finds that the ( T = 2, J P = 2 + state is bound with a binding energy of 10 to 40 MeV depending on the type of potential used and on the regularization range. Furthermore, for this state the decay width of the Δ is strongly reduced by binding effects and the Pauli principle. The reduction amounts to a factor of three to five.

Exchange currents in electric transitions and the rôle of siegert's theorem: A case study in deuteron photodisintegration

Abstract The role of the ηNN interaction is studied in the low-energy regime in η–deuteron reactions as well as in coherent and incoherent η photoproduction on the deuteron using a three-body model with separable two-body interactions. The three-body approach turns out to be quite essential in the most important lowest partial wave. Results are presented for differential and total cross sections as well as for the η-meson spectrum. They differ significantly from those predicted by a simple rescattering model in which only first-order ηN and NN interactions in the final state are considered. The major features of the experimental data of η photoproduction in the near-threshold region are well reproduced.

Investigation of a quasi-bound T = 2 (NΔ) state with increased lifetime of the Δ

Abstract The influence of the tensor force in the NN interaction on the zero proton-angle deuteron photodisintegration is discussed in detail. Standard NN potentials yield a cross section 30 to 40 % too large compared to experiment thus suggesting a somewhat too strong tensor force on which the cross section depends very sensitively. A recent potential of Holinde and Machleidt having a weaker tensor force improves remarkably the agreement though at higher energies the theoretical cross section is still systematically too high by about 10 to 20 %.

The ηNN-system at low energy within a three-body approach

Abstract The two-body photo break-up of the deuteron above pion threshold is calculated treating the final state interaction in a coupled channel approach with inclusion of Δ degrees of freedom. For the electromagnetic interaction nucleon, isobar and meson exchange currents are taken into account. Various observables (total and differential cross sections, γ-asymmetry and nucleon polarization) are influenced by the dynamical treatment of Δ degrees of freedom. Especially for the proton polarization it leads to an improved agreement with experiment. However, the situation for the shape of the differential cross section remains problematic.